The general term "atomic clock" has been loosely applied to any device that depends for its constancy of rate on the frequency of a spectral line, i.e., on the energy difference of two states, for the measurement of time intervals. In some cases they are those of an atom, e.g., the two states into which the ground state of cesium is separated in a magnetic field; in other cases they are those of a molecule, e.g., the vibration spectrum of ammonia.
Successful atomic clocks have been built employing a beam of cesium atoms. The beam is separated into two spin components, with the component made up of atoms in one of the two possible states, e.g. the spin-up state, being passed into a resonant chamber fed by an oscillator. The atoms will invert their spins and as a result, change state if the microwave frequency of the oscillator is equal to the frequency of resonance. A second separation of the beam then again gives two components with opposite spin; the relative intensities of these two may be measured and may be used to keep the oscillator in exact resonance. The period of oscillation is thus matched to the frequency of the spectral line, and the number of oscillations in a time interval may be counted by electronic means.
Such atomic frequency standards provide the most stable frequencies available. The cesium clock operated by the National Bureau of Standards requires the use of vaporized cesium metal obtained from an electric furnace. The atomic process involves a transition in the precession axis (hence electromagnetic field) of the outer electron of the cesium atom. The required radio frequency is 9,192,631,770 Hz (cycles per second) and the accuracy is to a variation of less than one part in ten to the thirteenth power.
National standards laboratories maintain large ensembles of such atomic clocks to form the legal bases of time within their countries. These clock ensembles (or "time scales") are compared internationally by the International Time Bureau (BIH), in Paris, France, to form International Atomic Time (TAI). The BIH also forms a time scale called UTC which is a compromise between pure atomic time (TAI) and "earth time" (UTI). Almost all nations base their official time on UTC shifted by an integer number of hours corresponding to the appropriate time zones. Each participating standards laboratory steers its realization of UTC to approximate the BIH realization.
Besides the national standards laboratories, many organizations need atomic clock accuracy for various systems. Examples include telecommunications systems, electronic navigation systems such as Loran-C, Transit, and the Global Positioning System, and scientific laboratories. There are now tens of thousands of atomic clocks in use in such applications.
Although atomic clocks can realize rate accuracies of one part in ten to the twelfth power, or better, these inaccuracies can accumulate to several tens of microseconds error in a year's time. Although this may seem insignificant, many systems need this level of accuracy and could use even more. Like all clocks, atomic clocks drift out of synchronism with a master clock. The difference is only in the size of the numbers, not their importance.
Atomic clocks are more stable in frequency than they are accurate in frequency. That is, once set, the frequency of a typical cesium beam standard changes only slightly (1.0e-13). As delivered, however, the cesium beam frequency might differ from the international standard by as much as +/-5.0e-12. Accordingly, it is desirable to have the capability of fine tuning adjustments (+/-3.0e-11) on atomic frequency standards. By such means, the frequency of the local clock could be adjusted or offset to agree more closely with the national standards or other system reference.
Moreover, the internationally recognized time scales are deliberately adjusted from time to time. At the end of each year it may be necessary to adjust the recognized time scales to account for fluctuations in the primary frequency standard. Of course, to maintain the usefulness of the atomic frequency standards, it is desirable to offset their outputs to conform to the national standard.